A liquid chromatograph mass spectrometer (LC/MS) having a liquid chromatograph (LC) and a mass spectrometer (MS) combined with each other normally includes an atmospheric pressure ion source using electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) or other methods to generate gaseous ions from a liquid sample. In an atmospheric pressure ionization mass spectrometer using an atmospheric pressure ion source, the ionization chamber in which the ions are generated is maintained at substantially atmospheric pressure, whereas the analysis chamber in which a mass separator (e.g. a quadrupole mass filter) and a detector are contained must be maintained in a high-vacuum state. To satisfy these conditions, a multi-stage differential pumping system is adopted, in which one or more intermediate vacuum chambers are provided between the ionization chamber and the analysis chamber so as to increase the degree of vacuum in a stepwise manner.
In the atmospheric pressure ionization mass spectrometer, a stream of air or gaseous solvent almost continuously flows from the ionization chamber into the intermediate vacuum chamber in the next stage. Therefore, although the intermediate vacuum chamber is maintained under vacuum atmosphere, the gas pressure in this chamber is relatively high (which is normally at approximately 100 [Pa]). One example of the system for efficiently transporting ions to the subsequent stage under such a relatively high gas pressure is an ion guide composed of a plurality of “virtual” rod electrodes arranged so as to surround an ion-beam axis, each virtual rod electrode consisting of a plurality of plate electrodes arranged at intervals in the direction of the ion axis (see Patent Documents 1-3). Such an ion guide is capable of efficiently converging ions and transporting them to the subsequent stage even under a high gas pressure, and therefore, is useful for improving the sensitivity of the mass spectrometry.
Regarding such a multi-stage differential pumping system, it is commonly known that, when ions are accelerated in the first-stage intermediate vacuum chamber, the energized ions collide with the residual gas and produce fragment ions. This function is called in-source collision induced dissociation (CID). By performing a mass spectrometry on the fragment ions generated by the in-source CID, it is possible to easily analyze the structure or other aspects of a substance.
Normally, for the in-source CID, different voltages are applied to the first and second electrodes, which are separately arranged in the traveling direction of the ions within the first-stage intermediate vacuum chamber, so as to create a direct-current potential difference between the two electrodes and accelerate the ions by the effect of an electric field having that potential difference. The efficiency of dissociating the ions in the in-source CID depends on the amount of energy given to the ions. Accordingly, in a conventional mode of in-source CID performed in an atmospheric pressure ionization mass spectrometer, the voltages applied to the electrodes are adjusted so that the intensity of an ion in question will be maximized. When the in-source CID should not be performed in the atmospheric pressure ionization mass spectrometer (i.e. when the fragment ions are unwanted), it is common that the voltages applied to the electrodes be controlled so that no acceleration of the ions occurs in the first-stage intermediate vacuum chamber.
However, this conventional system has the following problem:
When ions are introduced from the ionization chamber maintained at substantially atmospheric pressure into the first-stage intermediate vacuum chamber through a small diameter capillary and orifice or similar structure, the ions are cooled due to an adiabatic expansion. The cooled ions are more likely to be combined together due to the van der Waals force, forming a cluster ion (i.e. a mass of ions). When cluster ions are formed, unintended peaks appear on the mass spectrum, making the peak pattern of the mass spectrum complex and difficult to analyze. The adiabatic expansion also causes the ions originating from the sample to be combined with the molecules of the solvent in the mobile phase, making the peak pattern of the mass spectrum even more complex. The generation of a dimer, trimer or the like of the ions of the solvent in the mobile phase can also occur, which forms a background noise and deteriorates the quality of the chromatogram.
None of the conventional atmospheric pressure ionization mass spectrometers have barely taken into account the influence of the background noise due to the cluster ions or the like created inside the first-stage intermediate vacuum chamber in the previously described way, and no active efforts for reducing such a noise have been made thus far. This problem is particularly noticeable when the voltages applied to the electrodes are adjusted so as to maximize the intensity of the target ions for the sake of the in-source CID. Under this condition, although a high dissociating efficiency is achieved, a relatively large amount of cluster ions are often produced, which may possibly deteriorate the quality of the mass spectrum or chromatogram, making it difficult to perform a qualitative and/or structural analysis of the substance of interest.